How is he after he blows up (like the next day or two after he's had a chance to cool down)?
I know that I sometimes get caught with the jealous bug every once in awhile... My friend is very attractive, is always very easy to talk with, and with people she knows is also physical (is that the right word?) in that she likes to give hugs to people either in greeting or saying goodbye, getting massages on her shoulders, etc. (and with either sex). At times I think that guys see this and think this is an open invitation to flirt (sometime heavily like touching her on her leg when they talk). And it's times like those that I tend to get jealous and blow up, give her a cold shoulder, or even just walking away. But usually after I've had a chance to analyze the situation with a cooler head, I can see that it's the guys that are reading mixed signals from her (she thinks she is just being friendly whereas guys think she is flirting) and also realize that in the end, I'm the one she keeps going out with, truly flirts with, etc.
I think in some ways women tend to be more outgoing and friendly like that and may just get read wrong by men... How often have you seen girls give a hug to everyone in a crowd of friends vs. the number of times you've seen men act the same way? A man might interpret this as flirting while the woman just feels that she is being friendly with no romantic inclinations. If you talked with him, esp. after he's had a chance to cool down, you may be able to get to the root of what is triggering this in him. And if it's like me and my gal, perhaps just let him know that deep down, he is the only one that matters. Like in The Drifters' song "Save the Last Dance For Me" where the guy tells his woman she is free to socialize and dance with other guys, as long as she remembers who is going to be taking her home (and actually a very touching song when you know the back-story, that the song's writer had polio and was wheelchair bound for his wedding so he let his actress wife be free and happy with their guests). A good way to at least attempt to push down those jealous feelings when they erupt and to remember who you love truly is.